1. Field of the Invention
The present invention relates to substrate processing such as film formation performed using a chemical vapor deposition apparatus and fine processing performed using a dry etching apparatus, and more particularly relates to a technique for improving uniformity of processing.
2. Prior Art
In recent years, there has been a trend where the degree of integration of semiconductor integrated circuit devices is further increased whereas the power consumption thereof is further reduced. Meanwhile, costs in semiconductor integrated circuit device fabrication have been reduced by increase in the diameter of semiconductor substrates and the like. In order to reduce pattern dimensions of an element in such a semiconductor integrated circuit device while increasing the diameter of a semiconductor substrate, it is necessary to suppress variation in the thickness of an insulation film which is a member of the semiconductor integrated circuit device in fabrication process steps and to uniformly perform dry etching to an entire surface of the substrate.
FIG. 6 is a cross-sectional view illustrating an exemplary known chemical vapor deposition (CVD) apparatus for forming a thin film such as a silicon oxide film and polysilicon on a semiconductor substrate. With the recent trend of increase in the diameter of semiconductor substrates, the use of such a single-wafer-processing apparatus as the CVD apparatus of FIG. 6 is the mainstream. FIG. 6 shows a thermal reaction type CVD apparatus.
The known chemical vapor deposition apparatus of FIG. 6 includes a reaction chamber 111 for forming a film on a semiconductor substrate 107, a shower head 102 for supplying a source gas 101 used in film formation, a gas inlet 105, connected to upper part of the shower head 102, for introducing the source gas 101 to the shower head 102, a substrate support 104, disposed in the reaction chamber 111, for placing the semiconductor substrate 107, and an outlet 106 connected to the reaction chamber 111 and a pump (not shown) for exhausting gas in the reaction chamber 111.
A gas diffusion plate 108 in which a large number of small through holes 103 for discharging the source gas 101 to the inside of the reaction chamber 111 are provided is attached to a (lower) side of the shower head 102 facing the substrate support 104. The gas diffusion plate 108 forms a hollow portion together with a main body of the shower head 102. The substrate support 104 includes a heater for adjusting a temperature of the semiconductor substrate 107 therein and the semiconductor substrate 107 is disposed on the substrate support 104 so as to face the gas diffusion plate 108.
The gas diffusion plate 108 can be also formed as a united body with the shower head 102. In such a case, however, processing and maintenance of the gas diffusion plate 108 would become complicated. Therefore, the gas diffusion plate 108 is usually a removable separate member.
When a thin film is formed using the known chemical vapor deposition apparatus having the above-described structure, first, the semiconductor substrate 107 is placed on the substrate support 104 in the reaction chamber 11I and is heated to a predetermined temperature. Then, while exhausting the reaction chamber 111 through the outlet 106 using the pump, the source gas 101 which is necessary for forming a film is introduced into the reaction chamber 111 through the gas inlet 105, thereby forming a thin film on the semiconductor substrate 107 in the form of a wafer.
The gas diffusion plate 108 has a disc shape and, in the gas diffusion plate 108, the small through holes 103 with a diameter of about 0.5 mm are formed so as to spread uniformly throughout substantially a whole surface of the diffusion plate 108. As shown in an enlarged view of FIG. 6, the longitudinal section of each of the through holes 103 have the same shape at any point from the gas inlet to the outlet. With the gas diffusion plate 108, the source gas 101 introduced into the hollow portion from substantially center part of the shower head 102 through the gas inlet 105 can be discharged into the reaction chamber 111 so as to be diffused uniformly in the horizontal direction. Therefore, the thickness of a thin film deposited on the semiconductor substrate 107 can be made uniform. A chemical vapor deposition apparatus using the gas diffusion plate is disclosed, for example, in Japanese Laid-Open Publication No. 2000-273638. Moreover, for dry etching apparatuses, a structure of an etching gas diffusion plate formed with improved etching uniformity is disclosed in Japanese Laid-Open Publication No. 6-204181.
However, as the size of elements in the semiconductor integrated circuit devices is further reduced in future and more precise process control is required, it becomes difficult to deposit, using the shower head 102 in the known chemical vapor deposition apparatus of FIG. 6, a film with sufficiently high uniformity of film thickness for fabricating a semiconductor integrated circuit device with high yield.
The through holes 103 are formed substantially throughout a whole surface of the gas diffusion plate 108 with a substantially uniform density. In this case, the source gas 101 is supplied more around inlets of ones of the through holes 103 located in the center part of the gas diffusion plate 108 and less around inlets of some other ones of the through holes 103 located at greater distance from the gas inlet 105 in peripheral part of the gas diffusion plate 108. Thus, it is assumed that the flow rate of a source gas discharged into the reaction chamber 111 varies depending on the locations of the through holes 103 and a nonuniform distribution of the thickness of a thin film is caused depending on the location of the semiconductor substrate 107 placed so as to face the gas diffusion plate 108.
It is desirable that during film formation using the chemical vapor deposition apparatus of FIG. 6, the pressure of the source gas 101 ideally becomes uniform at the whole surface of the gas diffusion plate and the flow rate of gas flowing through the through holes located around the center part of the shower head 102 and the flow rate of gas flowing through the through holes located in the peripheral part are the same. However, in actual fact, the pressure of the source gas 101 in the shower head 102 is lower in the peripheral part than in the center part for the above-described reason and thus the flow rate of gas flowing through the through holes located in the peripheral part becomes lower than the flow rate of gas flowing through the through holes located around the center part. As a result, gas is supplied less in peripheral part of the semiconductor substrate 107 and more around center part of the semiconductor substrate 107 and thus the thickness of a thin film formed on the semiconductor substrate 107 becomes small in the peripheral part of the semiconductor substrate 107.
The technique for improving nonuniformity in a single-wafer-processing apparatus for processing a substrate is disclosed in Japanese Laid-Open Publication No. 6-204181. The disclosed technique is for use in a dry-etching apparatus. According to the disclosed technique, in order to achieve uniform density for a reaction gas flowing from an entire surface of an electrode plate, small through holes are formed in center part of the electrode plate and large through holes are formed in peripheral part of the electrode plate. That is, through holes are formed so that a distribution of the size of the through holes becomes nonuniform. According to the method disclosed in Japanese Laid-Open Publication No. 6-204181, by properly setting a distribution of size of the through holes, etching uniformity can be improved to 3.3%. In general, a maximum etching rate and a minimum etching rate in a wafer are compared to each other and etching uniformity is obtained by using 100×((variation in etching rate)/(average etching rate))/2 (%), i.e., 100×((maximum etching rate−minimum etching rate)/(average etching rate for measurement points))/2 (%). However, to fabricate a fine semiconductor integrated circuit having a dimension of ¼ μm or less, the above-described etching uniformity is not sufficient, and higher uniformity is required. Moreover, it can be assumed that if the technique is used for a gas diffusion plate in a chemical vapor deposition apparatus, uniformity of flow rate distribution of a source gas can not reach a sufficiently high level for fabricating a semiconductor integrated circuit.
In addition, when through holes and the distribution of the through holes are adjusted to improve uniformity of substrate processing by the above-described method, as disclosed in Japanese Laid-Open Publication No. 6-204181, uniformity of substrate processing might be deteriorated to be 10% or more even in the case of fine adjustment. That is, according to the known method, uniformity sensitively varies with respect to adjustment of the through holes and the distribution of the through holes and therefore it is very difficult to achieve uniformity of 3% on an average or higher uniformity.